How to handle db constraint violations in the user interface?

Question

We implement the majority of our business rules in the database, using stored procs.

I can never decide how best to pass data constraint violation errors from the databa

Answer 1

A stored procedure may use the RAISERROR statement to return error information to the caller. This can be used in a way that permits the user interface to decide how the error will appear, while permitting the stored procedure to provide the details of the error.

RAISERROR can be called with a msg_id, severity and state, and with a set of error arguments. When used this way, a message with the given msg_id must have been entered into the database using the sp_addmessage system stored procedure. This msg_id can be retrieved as the ErrorNumber property in the SqlException that will be raised in the .NET code calling the stored procedure. The user interface can then decide on what sort of message or other indication to display.

The error arguments are substituted into the resulting error message similarly to how the printf statement works in C. However, if you want to just pass the arguments back to the UI so that the UI can decide how to use them, simply make the error messages have no text, just placeholders for the arguments. One message might be '"%s"|%d' to pass back a string argument (in quotes), and a numeric argument. The .NET code could split these apart and use them in the user interface however you like.

RAISERROR can also be used in a TRY CATCH block in the stored procedure. That would allow you to catch the duplicate key error, and replace it with your own error number that means "duplicate key on insert" to your code, and it can include the actual key value(s). Your UI could use this to display "Order number already exists", where "x" was the key value supplied.